This invention relates to vertical cavity surface emitting lasers (VCSELs) and more particularly to VCSELs with dielectric mirrors.
Vertical cavity surface emitting lasers (VCSELs) include first and second mirror stacks formed on opposite sides of an active area. The active area includes one or more quantum wells capable of generating light as electrical carriers are supplied. Each mirror stack includes a plurality of pairs of mirrors designed to reflect a portion of light generated in the active area back into the active area for regeneration. The pairs of mirrors in the mirror stacks are formed of a material system generally consisting of two materials having different indices of refraction to provide the reflectivity. Two types of mirror stacks are prevalent in the art: semiconductor distributed Bragg reflectors (DBRs) formed using relatively expensive and complex epitaxial growth; and dielectric mirror stacks, which can be formed using much simpler chemical and physical deposition techniques. Also, the epitaxially grown DBR mirror stacks must be chosen to be easily lattice matched to the other portions of the VCSEL, which severely limits the reflectivity that can be obtained. Because of this limitation on reflectivity, epitaxially grown DBR mirror stacks generally contain twenty or more pairs of mirrors (pairs of layers).
In conventional VCSELs in the 760 nm to 1050 nm range, conventional material systems such as AlGaAs perform adequately. However, for VCSELs outside of this range, other material systems, whose overall performance is poorer, must be used. For example, longer-wavelength light can be generated by using a VCSEL having an InP-based active region. When an InP-based active region is used, however, the epitaxial DBRs lattice matched to the supporting substrate and the active region do not provide enough reflectivity for the VCSELs to operate because of the insignificant difference in the refractive indices between the two DBR constituents. Dielectric mirror stacks can be used instead, but they suffer from poor thermal conductivity. Since the performance of these long-wavelength materials is very sensitive to temperature, the thermal conductivity of the mirror stacks is very important.
At least two different embodiments of VCSELs have been suggested in an effort to overcome the thermal conductivity problem. In a traditional short-cavity, large-diameter-mirror VCSEL, almost all the heat must flow vertically through a bottom mirror. But if the bottom mirror has a modest thermal conductivity, i.e., if it is dielectric, the vertical heat flow will be poor. In a long-cavity, small-diameter-mirror VCSEL, heat can also flow laterally (or diagonally), bypassing the bottom mirror. However, in the long-cavity, small-diameter-mirror VCSEL the reflectance of the optical field, which spills over laterally beyond the beam-waist spot size (approximately the lateral dimension of the active area), is decreased. This decrease in reflectance reduces the efficiency and operating performance of the VCSEL. If the mirror diameter is increased, to capture the optical field spill-over, then the diagonal heat flow will be cut off or seriously reduced.
Accordingly it is highly desirable to provide VCSELs with mirror stacks that rectify these shortcomings.
It is an object of the present invention to provide VCSELs with improved mirror stacks.
It is another object of the present invention to provide VCSELs with improved mirror stacks that are easier to manufacture.
It is another object of the present invention to provide VCSELs with improved mirror stacks that do not require expensive and complicated epitaxially grown mirrors.
It is still another object of the present invention to provide VCSELs with improved mirror stacks that are tailored to enhance a desired or single transverse mode of operation.
It is a further object of the present invention to provide VCSELs with improved mirror stacks that provide good reflectivity and thermal conductivity with less complexity and costly manufacturing techniques.
To achieve the objects and advantages specified above and others, a segmented-mirror vertical cavity surface emitting laser (VCSEL) and methods of fabrication are provided. The segmented-mirror VCSEL includes an active portion with an active region having at least one quantum well and a lateral dimension. A first dielectric mirror stack is positioned on a first opposed major surface of the active portion and extends laterally beyond the lateral dimension of the active region. A second dielectric mirror stack is positioned on the opposed major surface and extends laterally beyond the lateral dimension of the active region. The second dielectric mirror stack includes portions defining first and second reflectance zones with mirror pairs in at least one of the portions being segmented to provide a first reflectivity and a first thermal impedance in the first reflectance zone, a second reflectivity lower than the first reflectivity in the second reflectance zone, and a second thermal impedance lower than the first thermal impedance in-the second reflectance zone.
In a preferred embodiment, the active portion is formed by epitaxially growing heat/current spreading layers on opposed sides of an active region. The active portion is grown on a sacrificial support. A dielectric mirror stack is deposited on one surface of the active portion so as to define the first and second reflectance zones. At least some of the layers of the dielectric mirror stack are patterned to provide segmented layers which cooperate to provide the different reflectances and thermal impedances. A base is affixed to the dielectric mirror stack and the sacrificial support is removed. A dielectric mirror stack is then deposited on the opposed surface of the active portion. In some applications the segmented layers can be tailored to enhance a desired mode of operation. For example, the segmented layers can be tailored to enhance a TEM00 mode of operation and to discriminate against higher order modes of operation.